They are “technically correct” measurements since they are a valid prefix, and could be used if you wanted. but they are very infrequently used in any industry. Since most of the time measurements are better served by higher precision (just using Meters) or need no precision at all over long distance (switch to kilometers), no need for excess measurement types unless necessary

Valid, but rarely used, as it’s usually just as fast to say “two hundred meters” instead of “two hecto meters”.

However, those prefixes have other (non-SI) uses. A hectare is common way of referring to a 100x100 meter area. And a decare is 10 ares, i.e. 0.1 hectare.

No, some measurements just aren’t used, even when they’d be a good fit.

Like lengths. We never use anything above km. Even for things like space, we say “million km” rather than gigametre.

The closest we come to hectometre is hectare, which is used for land area.

I can only tell you about how we do it here in the Netherlands, but very rarely.

We have highway location markers (in the form of a little pole and a sign with some markings on them) every hundred meters. Their official name is “afstandspaal” but people generally call them “hectometerpaal” because they denote the distance along a highway in hectometers.

When you hear about traffic jams, speed cameras, or car accidents on the radio, they’ll usually announce highway number and hectometer pole number so drivers know where to look.

The metric tonne (1000kg) is used rather than a megagram even though megagram is more fun to say. This is for historic reasons, mostly. Having Mg (megagrams) and mg (milligrams) next to each other would probably also just confuse people.

Hectares (100x100m, or 10000m²) are used to denote area for things like farms. Centiliters and deciliters are used for very specific purposes (medical and such).

As for general distance: meters and kilometers work fine. Same with grams and kilograms. Smaller measurements like centimeters and millimeters are also common (or milliliters and liters, rarely centiliters but you do see them in some recipes).

Funnily enough, we (although I notice it mostly among older people) do use ons and pond (ounce and pound), although an ons is 100 grams and a pond is 500 grams. Just like with the tonne, we adjusted them to make dealing with SI units easier; we did the same with the mijl (mile) and other units similar to the imperial name as a transition to the normal metric system. Technically speaking, you’re not allowed to sell advertise products by the ons or pond anymore (thanks, IJkwet of 1937) but if you ask anyone at market for a pound if cheese, they’ll know how much to get you.

Lastly, there are a few use cases where the inch is used. Screens are measured in inches for reasons I’m not entirely sure about (probably British and American exports). This leads to some rather silly phone spec sheets, listing width/length/height in centimeters and a screen diagonal in inches. We don’t even use a Dutch word for inch!

In France “hectare” (10 000m²) is used for fields and burning forest. Beside that deca or hectometers are never used

As an American who has gotten very used to metric units in studying engineering, the general rule I picked up is that you typically only change units every three orders of magnitude. So 8 decameters would typically be expressed as 80 meters, maybe 0.08 kilometers. Decameters and hectometers are a thing, but they’re not common units. Even centimeters don’t see much use compared to millimeters.

From my experience in Norway, these are typical in context of daily speech:

Weight (gram): tonne (a substitute name for Mg (Mega)), kg, hg, g, mg, μg (mostly in medicine)

Distance (meter): mil (10 km), km, m, dm (kinda rare), cm, mm

Volume (liter): l, dl, cl, ml

In my experience, the deca-predix is very rarely used. Most of the missing prefixes are just substituted for numbers, i.e. saying “a thousand kilometers” is much more common that “a megameter”. Of course, this differs depending on context, as a lot of the prefixes become more common within scientific fields where the sizes are common.

On a separate note, even the numbers can be a bit inconsistent. It has bothered me that it’s often common to say “a thousand milliard” instead of “one billion” (also note that we use the long scale).

in school in austria we teach dezimeter (tenth of a meter)

It depends on the situation, sometimes they are really handy but most of the time we stick to kilo, centi and mili.

Where i live, Hecto (100x) is used, for example to measure distances and areas for big properties. 1 hectometro equalls 100 m, or 1 hectarea (hm^2) equals 10000 m^2.

Also, it is widely use for pressure, cause 1 atm is 1013 hPa

Decameters are used but for special situations, like quantifying natural gas consumption

Yes, they are used, but typically in specialized applications which is why you don’t see them every day.

In Romania we use them for measuring areas. An “ar” is 100m^2 or a square decameter, and a hectare is a 10000m^2 or a square hectometer.

In Italy we use hectograms (“ettogrammi”, “etti” for short) in day to day life when buying groceries. You don’t ask for 200 grams of ham, you just ask for 2 etti.

In Poland: decimeters are sometimes used (I have been ordering cut sheet metal priced by square decimeters) , I have not seen decameters in use. Hectopascales are often used in weather reports. Decagrams are often used when buying food where these amounts make most sense (meat, candy).

The ‘more exotic’ prefixes are usually only used with some specific SI units and in very specific contexts.

In America, I’ve seen nurses and diabetics use deciliters in reference to medication or concentration before.

Sweden uses decimeters and deciliters